1. Field of the Invention
The present invention relates to a blind stitch sewing machine and a blind stitching method. More particularly, the present invention pertains to a blind stitch sewing machine and a blind stitching method whereby it is possible to employ a straight needle used in ordinary household sewing machines which is readily produced with high accuracy and at low cost and which sticks in a work stably with a relatively great piercing force and minimizes the possibility that the needle will be deformed or the pointed tip thereof will be damaged during blind stitching. It is therefore possible to obtain a beautiful and high-quality sewn product which has no spotted stitches and it is also possible for laymen such as housewives to perform a blind stitching operation satisfactorily without the need for a high level of skill in sewing and it is unnecessary to pay any more attention to the condition of the needle than is needed for ordinary household sewing machines.
2. Related Art Statement
Blind stitch sewing machines employing a curved needle have heretofore been known [e.g., Japanese Patent Post-Exam Publication Nos. 39-28960 (1964), 43-26076 (1968), 45-20026 (1970), 45-23980 (1970) and 45-37268 (1970), Japanese Patent Laid-Open (KOKAI) No. 58-105786 (1983), U.S. Pat. Nos. 3,760,747, 3,986,469, 4,043,284 and 4,416,207].
This type of conventional blind stitch sewing machine has a driving shaft, a needle activated to perform a circular reciprocating motion by the driving shaft, a looper driven by the driving shaft to perform a somewhat elliptic reciprocating motion in relation to both the needle and the sewing thread such that the looper is inverted at two ends of an elliptical path, a needle plate and a presser foot which clamp a work therebetween, a work raising member which raises a part of the work through an opening provided in the needle plate, and a work holding-down member which holds down the raised part of the work, so that a curved needle is circularly stuck into the raised part of the work, while the thread is caught with the looper so as to form a loop, and the curved needle is entwined with the loop, thereby performing blind stitching by means of a single-yarn chain stitch.
Referring to FIG. 10, a curved needle 3001 used in the above-described prior art has an overall circular configuration. The pointed tip 3002 of the curved needle 3001 is eccentric with respect to the trunk 3003 of the needle 3001 and extends along a circular arc defined by the outer periphery of the curved needle 3001 so that the gap between the tip 3002 and the pointed upper end of the work raising member (not shown) is constant. Therefore, the curved needle 3001 has the advantage that it can pierce the work readily and smoothly.
On the other hand, the curved needle 3001 suffers from the following disadvantages. Namely, the production of such a curved needle is extremely difficult and troublesome. It is likely that there will be variations in the accuracy of needles. The production cost of curved needles is about 5 times that of straight needles for use in ordinary household sewing machines. Since the needle is curved, upon entering a workpiece, the piercing force is relatively weak, and the needle cannot stick in a work stably, so that the needle may be deformed during the blind stitching operation, which leads to spotted stitches and hence deteriorates the quality of the sewn product. In addition, the tip of the curved needle is readily damaged, so that it must frequently be replaced with a new one. Thus, the prior art requires a high level of skill in sewing, so that it is difficult for laymen to perform a satisfactory sewing operation and it is necessary in order to obtain a sewn product of high quality to pay special attention to the condition of the needle.